diff --git a/src/Makefile b/src/Makefile
index d2c9e11..a09ff54 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -56,7 +56,7 @@ HACKERBASE-OBJS=hackerbase.o testing.o listing.o cal-month.o		\
 	 util-utf8.o sgr-cell.o template-list-expander.o		\
 	 box-drawing.o util-list.o export-web-static.o util-dir.o	\
 	 dokuwiki.o racket-kwargs.o duck.o util-bst.o			\
-	 util-bst-bdict.o util-bst-ldict.o
+	 util-bst-bdict.o util-bst-ldict.o util-bst-lset.o
 
 GENDOC-SOURCES=gendoc.scm duck-extract.import.scm			\
 	util-time.import.scm util-csv.import.scm util-git.import.scm	\
@@ -526,3 +526,10 @@ UTIL-BST-LDICT-SOURCES=util-bst-ldict.scm duck.import.scm	\
 
 util-bst-ldict.o: util-bst-ldict.import.scm
 util-bst-ldict.import.scm: $(UTIL-BST-LDICT-SOURCES)
+
+UTIL-BST-LSET-SOURCES=util-bst-lset.scm util-bst.import.scm	\
+	racket-kwargs.import.scm util-bst-ldict.import.scm	\
+	util-list.import.scm
+
+util-bst-lset.o: util-bst-lset.import.scm
+util-bst-lset.import.scm: $(UTIL-BST-LSET-SOURCES)
diff --git a/src/util-bst-lset.scm b/src/util-bst-lset.scm
new file mode 100644
index 0000000..0a255d1
--- /dev/null
+++ b/src/util-bst-lset.scm
@@ -0,0 +1,65 @@
+
+(declare (unit util-bst-lset))
+
+(import duck)
+
+(module*
+ util-bst-lset
+ #:doc ("Reimplementation of old lset using new BST backend.")
+ (
+  make-lset
+  lset?
+  lset-empty?
+  lset-member?
+  lset-count
+  lset-add
+  lset-remove
+  list->lset
+  lset->list
+  lset-merge
+  lset-intersect
+  lset-subtract
+  lset=?
+  )
+ (import scheme
+	 util-bst
+	 racket-kwargs
+	 util-bst-ldict
+	 util-list)
+
+ (define* (make-lset (equality? equal?))
+   (make-bst 'lset equality? ldict<?))
+
+ (define lset? (bst? 'lset))
+
+ (define lset-empty? bst-empty?)
+ (define lset-member? bst-contains?)
+ (define lset-count bst-count)
+
+ (define (lset-add ls e)
+   (bst-set ls e #t))
+
+ (define lset-remove bst-remove)
+
+ (define* (list->lset lst (equality? equal?))
+   (foldl lset-add (make-lset equality?) lst))
+ 
+ (define lset->list bst-keys)
+
+ (define (lset=? ls1 ls2)
+   (bst-equal? ls1 ls2))
+
+ (define (lset-merge ls1 ls2)
+   (foldl lset-add ls1 (lset->list ls2)))
+
+ (define (lset-intersect ls1 ls2)
+   (list->lset
+    (filter (lambda (el)
+	      (lset-member? ls2 el))
+	    (lset->list ls1))
+    (bst-EQ? ls1)))
+
+ (define (lset-subtract ls1 ls2)
+   (foldl lset-remove ls1 (lset->list ls2)))
+
+ )
diff --git a/src/util-bst.scm b/src/util-bst.scm
index 7a16475..93f97a7 100644
--- a/src/util-bst.scm
+++ b/src/util-bst.scm
@@ -10,6 +10,10 @@
   make-bst
   bst?
 
+  bst-EQ?
+
+  bst-count
+
   bst-empty?
   bst-ref
   bst-contains?